Importance of layers
Layers create easily alterable adjustments/additions/subtractions to the original background image.
The layer stack is like a stack of transparencies. Each can add or subtract a color, part of an image, etc. Use Layer->New Adjustment Layer->Levels to add a Levels adjustment layer.
Ok the New Layer dialog box. Slide the highlights slider to the left to temporarily brighten the image and hit Ok.
Now you have an adjustment layer that lightens the image.
Be sure the layer mask is selected.
Select your paint brush , and be sure the foreground color is black. Make your brush large, and paint on the displayed image.
Notice that while you paint on the image, it is pixels in the mask that are turning black, and as they turn black the block the effect of the adjustment layer. (The image gets darker again, as if there were no levels adjustment.)
Every adjustment layer has a mask, and, as you will see, they are EXTREMELY useful.
Each layer can be turned on and off with the “eye” icon.
The image that you see is the sum total of all the visible layers. At every layer, each pixel is evaluated to see what happens to it here, if anything.
The order of layers can be changed by dragging them, except the background layer.
The opacity of layers can be changed.
Layers can be clipped (Alt-left click on border between layers) so that certain layers only affect the ones immediately above them, and not the whole stack.
The Blend Mode of the layer can be changed to startling and very useful effect (try it when you have an hour).
